Okay well dad told me to f-off and fix it myself (in the nicest possible way).

I stripped the engine (process of elimination). As she came apart I kept cranking it and the knocking turned out to be several different knocks.

The bearings are sitting tight (well as tight as they ought to be). I put a size 27 spanner on the crank pulley (engine head off) nut and moved the crank back and forth, I placed a screwdriver (gently) on each piston and put my ear to the top of the screw-driver. As I rocked the crank the knocking was very audible through the screw-driver through all of the pistons. It seems the knocking is coming from the piston head and the pin securing the piston head to the conrod.

What is this pin called? Gudgeon pin comes to mind but I don't know what it really is.

The pistons are oversized with a '100' stamped on the tops. Is this apparent knocking normal?

The knock isn't huge but it is audible and in my mind it isn't right.

What shall I do?